Processes for producing alumina trihydrate by alkaline digestion of bauxite ore usually comprise, among others steps, a digestion step, a separation step and a precipitation step.
The digestion step is for extracting alumina from the bauxite ore by bringing it into contact with a sodium aluminate liquor, the digestion leading to the formation of a slurry including an enriched sodium aluminate liquor comprising dissolved alumina and insoluble residues formed of particles of undissolved bauxite ore, the insoluble residues being also referred to as red mud.
The separation step is for treating the slurry obtained in the digestion step, in order to separate the enriched sodium aluminate liquor from the insoluble residues.
The precipitation step, often referred to as a decomposition step, is for treating the enriched sodium aluminate liquor separated from the insoluble residues, in order to precipitate alumina in the form of alumina trihydrate.
The process schematically illustrated in FIG. 1, represents the usual process steps for producing alumina trihydrate from bauxite ore of a known Bayer process
Referring to FIG. 1, the bauxite ore 10 is fed to a grinding step 12, in order to crush the ore usually in the presence sodium aluminate liquor. The resulting slurry 14 is fed to a desilication step 16. After desilication, a slurry of desilicated liquor 18 and bauxite ore is preheated in a pre-heating step 20 and brought into contact with a sodium aluminate liquor which is provided from a fresh sodium aluminate liquor stream not shown and from a sodium aluminate liquor recycled stream 22. The preheated slurry 24 of sodium aluminate liquor and bauxite ore is fed to a digestion step 26 in a digestion chain where the digestion is carried out under pressure and at high temperature. The digestion chain is usually comprised of a series of autoclaves in which the slurry circulates. During the digestion process, a slurry comprising of enriched sodium aluminate liquor and insoluble residues is obtained. During the digestion, the slurry passes through heat exchangers not shown allowing the recovery of heat into the preheating step 20. The slurry 28 resulting from the digestion is then depressurized in a step 30. The depressurized slurry 32, which still includes enriched sodium aluminate liquor and insoluble residues, is then sent to a separation step in order to separate the enriched sodium aluminate liquor from the insoluble residues. The separation step typically includes a decanting or settling step 34 in a gravity settling vessel in which the insoluble residues are separated from the enriched sodium aluminate liquor by gravitation. The settling tank is generally under pressure. The insoluble residues are removed from the bottom of the settling tank in the form of red mud 36 while the enriched sodium aluminate liquor, usually referred to as clarified liquor, is separated from the red mud in an overflow stream 38 of said settling tank. The red mud 36 is then washed with water 40 in countercurrent washers 42, in order to recover sodium aluminate. The overflow 44 of a first washer which has a very high content of sodium aluminate can be recycled back to the depressurization step 30. Alternatively, the overflow 44 of the first washer, can be passed through an auxiliary filtration step 66, and the filtrate stream 67 obtained during said filtration step can then be sent to a precipitation step 52 described hereafter. The washed red mud 46 is sent to a disposal area. The overflow stream 38 of clarified liquor is then passed through a filtration step 48, usually referred to as “red filtration” or “security filtration”. A clarified filtrate of enriched sodium aluminate liquor is obtained, often referred to as supersaturated aluminate liquor. A stream 50 of the enriched sodium aluminate liquor separated from the insoluble residues, based essentially on the filtrate obtained from the filtration step 48, is sent to further steps for recovering the alumina as smelter grade alumina. These steps include a precipitation step and a calcination step. Generally, before being precipitated, the supersaturated aluminate liquor stream 50 is further cooled to increase supersaturation of dissolved sodium aluminate. The precipitation is carried out in a decomposition chain that includes a series of precipitators 52, where the filtrate 50 is progressively cooled for precipitating the alumina trihydrate. The precipitation generally further includes a classification step carried out in classification circuit 54. The classification circuit is fed with a slurry of alumina trihydrate 53 which exits the precipitation circuit 52. A stream 56 of fine alumina trihydrate is separated in the classification circuit 54 and is recycled back to the precipitation circuit 52 as seed. On leaving the classification circuit 54, a depleted or spent liquor 58 is concentrated by evaporation 60 and the resulting concentrated liquor is sent back to the digestion step via the recycled stream 22, while the produced alumina trihydrate 62 is fed to a calcination step 64.
As illustrated in FIG. 1, the solid-liquid separation step generally requires a further filtration step 48 for removing most of the remaining particles of insoluble residue from the clarified liquor. The remaining particles being often very fine, a filtration additive or filter aid 68, such as lime or tricalcium aluminate, needs to be applied to the filtration cloth to prevent blockages and improve the filtration rate. The same apply to the auxiliary filtration step 66. The tricalcium aluminate is usually obtained from lime as raw material, but also consumes sodium aluminate and therefore reduces the conversion efficiency of the Bayer process. The equipment and the operation of the filtration step are therefore quite complex and result in high capital and operating costs.
In the known Bayer processes, the use of filtration after the gravity settler vessel makes sure that the supersaturated liquor produced during the separation step, and to be sent to the precipitation step, is of high purity. The impurities that that could be carried with said liquor into later steps of alumina recovery process, namely the precipitation and calcination steps, tend to reduce the purity of the resultant alumina and make the operation of the later alumina recovery stages more difficult. For instance, the presence of inorganic suspended solids in the supersaturated liquor is liable to lead to the contamination of the alumina by iron and lead to a product outside of the specification limits for smelter grade alumina
The overflow stream of clarified liquor flowing out of the gravity settler vessel usually has a concentration of very fine insoluble residue, that can be determined by turbidity measurement, that is in the order of magnitude of 100 mg/L. This necessitates the use of a filtration step in order to make sure that the alumina produced has an acceptable purity with respect to iron.
The alumina refining industry has, and is still, looking at improving the performance of the Bayer process separation step, and more particularly of the settling step of said separation step, with the aim to simplify the implementation and the operation of the filtration step and to reduce associated cost. These improvements are often obtained by developing a pre-treatment step prior to the settling step involving the use of different types of flocculants, with different dosage and/or with different mixing operating conditions.
U.S. Pat. No. 4,040,954 describes a process for settling suspended particles in a liquor supplied to a settlement vessel, said process comprising the use of flocculating agent and the control of the relative proportions of flocculating agent and supplied liquor by measuring the turbidity at different height in the settlement vessel. A turbidity measuring probe is automatically raised or lowered at a height of the settlement vessel in order to cancel out any shifting from the turbidity predetermined value, the control being carried out by acting on the supply of flocculating agent in response to any change of height of the probe.
International patent application WO 2012/003578 describes a pre-treatment step for adding flocculants to the slurry and for mixing the flocculants and said slurry to obtain a flocculated slurry. The mixing in the described pre-treatment step is done in a succession of mixing stages, by selecting a higher extent of mixing in an early stage than in a later stage. Such a pre-treatment step improves the performance of the settling step by producing an overflow stream out of the gravity settler vessel that has a lower concentration of solid particles.
Even if there are ways of improving the gravity settling step, such as by using the above-described flocculant pre-treatments, a filtration step is always used after the gravity settling step, since said filtration step acts as a safety net for preventing any production losses or product contamination in case of reliability issues encountered in the settling step and/or its pre-treatment.
There is a need to improve the separation step, by simplifying it and by reducing its related capital and operation costs, while maintaining the purity of the produced alumina and the reliability of operation to an acceptable standard.